The subject matter disclosed herein relates generally to power supply circuits for a load, and, more particularly, to a power supply circuit that will allow a load, such as a relay, to be operated over a wide input voltage range while providing a reduced current operational threshold.
Many applications exist for switching devices such as relays. In general, such devices typically include one or more contacts that can be opened or closed in response to energization of a relay coil. Both electromechanical and solid state relays are commonly available. Sizes and ratings of such devices vary widely, depending upon the needs of particular applications, and upon such factors as whether the relay powers significant loads or simply provides low-level feedback. Families of relays are currently available that are quite small in physical packaging, and that can be mounted on circuit boards, and other relatively small supports.
One difficulty associated with families of electrical devices such as relays is the large number of catalog numbers and associated relays that need to be manufactured and warehoused. Typically, a relay is designed for only one specific supply voltage. If you are a manufacturer, you want to offer a full product line, which means offering a large variety of relays with installed coils that operate at one supply voltage. If you are an integrator or an OEM, this mean that you need to have available a large selection of relays that operate at different voltages for your application's needs. Attempts to accommodate devices to operate on more than one supply voltage results in increased size, cost, and heat generation.
Another difficulty associated with certain relay applications resides in the presence of leakage current from upstream circuitry used to energize the relay coil. In certain relays, particularly in smaller size relays, such as those mountable on circuit boards and other small support structures, even low levels of leakage current can cause the relay coil to be energized when such energization is not desired, thereby causing the relay to open or close in an undesirable fashion, greatly reducing their reliability. Similarly, such leakage current can cause the coil to remain energized to a sufficient degree to prevent shifting of the contact or contacts upon removal of a control signal to the coil. In either case, the reliability of the relay and the signals produced by the relay can be jeopardized by the leakage current.
To mitigate the problems associated with leakage current, relay control circuits include under-voltage lockout circuits. These circuits are configured to force the relay coil into a non-energized state unless the input voltage exceeds a certain threshold. These lockout circuits, though preventing unwanted energization of the relay coil can be inefficient.
There is a need, therefore, for an improved technique for controlling relays and similar loads. There is a particular need for a power supply circuit that can accept a wide range of input voltages to allow fewer catalog numbers to be required, while at the same time, that can improve reliability by efficiently controlling coil energization based upon an input voltage value.